1. Field of the Invention
The present invention relates to a magnetic coating material and a method for preparing the same. More particularly, the present invention relates to hydrophilic magnetic metal oxide particles and a method for preparing the same.
2. Description of the Related Art
Electromagnetic wavelengths that can be absorbed by electromagnetic wave absorption materials are mainly divided into two ranges. One range is for commercial usage and has a frequency being about 30 MHz to about 3 GHz, which is usually studied by research institutes. The other range is for military usage to shield radars and has a frequency of about 3 GHz to above ten GHz.
Among the electromagnetic wave absorption materials, ferrite is a very important material. The chemical formula of ferrite is MFe2O4, where the M can be Fe, Mn, Co, Ni, Mg, Cu, Zn, or a mixture of the elements mentioned above. Generally speaking, if the ferrite has a better initial permeability, the ferrite has better electromagnetic wave absorption ability.
The electromagnetic wave absorption ability of the ferrite can be affected by the preparation method of ferrite. U.S. Pat. Nos. 4,003,840 and 4,023,174 state that ferrite powder has better electromagnetic wave absorption ability than ferrite clumps, which are formed by sintering. Furthermore, the particle size of the ferrite powder also affects the absorbed electromagnetic wavelength range. U.S. Pat. No. 4,879,075 has specified that the smaller the particle size of the ferrite powder, the larger the absorbed electromagnetic wavelength range. Ferrite powder that has a particle size of 50–100 nm has good absorption ability for the microwave range. Therefore, magnetic nanoparticles have good development potential in the application field of electromagnetic wave absorption.
Since the particle size of magnetic nanoparticles is quite small, the surface area of magnetic nanoparticles is very large. Thus, magnetic nanoparticles in solution tend to aggregate together to reduce surface area and thus the surface energy. If the magnetic nanoparticles aggregate together, the application value in electromagnetic wave absorption is reduced, too. Therefore, how to prevent the magnetic nanoparticles from aggregating together and how to maintain the magnetic nanoparticles in a dispersed state for a long time become a major research topic in magnetic nanoparticle research.
There are three major research directions to keep magnetic nanoparticles dispersed in solutions for a long time. The first research direction is to study how to control the electrostatic interaction among the magnetic nanoparticle surfaces. The second research direction is to study how to control the affinity between magnetic nanoparticles and solvent. The third research direction is to study how to control the magnetic nanoparticle morphology to build up the steric hinderance effect. For example, S. S. Papell combines the first and the second principles to disperse the ferrite nanoparticles in various solvents by utilizing surfactants (NASA Technical Note, Vol. NASA-TN-D-4676).
Most of the developed dispersion reagents use dipole interaction to disperse the magnetic nanoparticles. However, the interaction between the dispersion reagent and nanoparticles is weak and easily affected by environmental changes, and the dispersion efficiency is thus reduced.